Ingo Molnar wrote:
because userspace does not use it normally, while with %gs we'd switch between glibc's descriptor [which must be shadowed by the CPU] and the kernel's descriptor [which must be shadowed by the CPU too] - hence causing a constant reloading of the shadow register.
Well, that means the only operation which would be different would be the pop %fs at the end, since only it would end up loading a null selector. All the other operations would presumably take just as long.
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